A broadband wireless access system supports a sleep mode to minimize power consumption of a mobile station. The sleep mode is comprised of a sleep interval for preventing power consumption in an interval during which a mobile station does not communicate with a base station and a listening interval for receiving an indication of whether data traffic is present between sleep intervals from the base station.
The length of the listening interval is fixed by a sleep request message (e.g., “MOB_SLP-REQ” message) and a sleep response message (e.g., “MOB_SLP-RSP” message). During the listening interval, the mobile station receives indications as to whether downlink traffic of the mobile station is present and whether ranging for maintaining uplink (UL) communication and a proper downlink (DL) coding type according to signal quality should be performed, through a traffic indication message (e.g., “MOB_TRF-IND” message) transmitted in broadcast form from the base station.
Meanwhile, a channel quality indicator (CQI) reporting procedure of a sleep-mode mobile station is as follows.
The sleep-mode mobile station may report a CQI to a slot allocated through a CQI channel (CQICH) allocation information element (CQICH_Allocation_IE) during a normal mode according to a designated period, a report type, a feedback type, etc. Here, a stop CQI allocation flag of a sleep control message including the MOB_SLP-RSP message, a ranging response (RNG-RSP) message, a downlink sleep control extended subheader, etc. may indicate ‘1 ’ and at this time, the mobile station stops reporting the CQI allocated through the CQICH_Allocation_IE.
When resource allocation for CQICH transmission is stopped as described above, if a sleep-mode mobile station of a broadband wireless access system recognizes that downlink traffic is present, the mobile station transmits a bandwidth request message for reporting the CQI to the base station using a contention-based random access method. In this case, if collision occurs during random access, a backoff process is implemented and thus transmission of the CQI is delayed. Accordingly, since the base station can not acquire information about a downlink channel state at a corresponding time point, the base station can not apply an accurate adaptive modulation and coding scheme to data transmitted to a corresponding mobile station.
Moreover, even though the mobile station periodically transmits the CQI, the same problem occurs if a time point when the mobile station recognizes that traffic is present does not coincide with a CQI reporting time point.